1. Field of the Invention
The present invention relates in general to the field of autopilot control systems, and more specifically, to the field of servo control devices that can automatically control an aircraft.
2. Background of the Related Art
An autopilot for a typical small, piloted aircraft is an aid to the pilot and functions to reduce the pilot's workload, reducing fatigue and enabling the pilot to concentrate on other tasks such as navigating, checking the instruments, or simply enjoying the flight. Such autopilots may handle only one axis, such as heading, or may handle all control surfaces and may be coupled, as desired, to radio or GPS navigation aids to enable fully automated control between or among one or more geographical points according to a preset plan.
A typical autopilot includes a control logic unit that receives pilot commands and interfaces with flight sensors and possibly radio navigation systems. In some systems, some of the flight sensors may be included in the same module with the control logic unit. The output of the control logic unit drives one or more servos. The servo is the device that receives electrical commands from the autopilot control logic unit and translates these commands into mechanical motion which in turn drives an associated aircraft control surface such as the ailerons or elevator.
Autopilots for piloted aircraft have special needs with respect to servo devices. The servo should be extremely reliable and pass stringent safety requirements. The safety requirements include a failure mode analysis wherein single or sometimes double failures shall not result in the unsafe or uncontrollable operation of the aircraft. This typically results in a requirement for several independent ways to remove or decouple the servo from the aircraft control mechanism.
In addition to stringent safety requirements, the servo should be lightweight, cost competitive, simple to install, and should be relatively easy to produce even at relatively low production volumes. This generally implies a heavy reliance on off-the-shelf standard components to make the servo, especially with respect to the motor.
Since autopilots are frequently added after manufacture of the aircraft, the autopilot and servo should preferably be easy to install in the field by service shops or by do-it-yourself mechanics and “home-built” aircraft builders, often with minimal documentation or “factory” installation designs.
A further desirable property is an easy interface to a digital autopilot control logic unit. Autopilots of recent design rely heavily on digital processing of sensor signals including rate gyros and accelerometers. It is thus, desirable that the servo present a simple interface to the digital autopilot control logic unit.
Existing servos typically utilize DC servomotors which require servo amplifiers. However, these DC servomotors and servo amplifiers are frequently too large or fall short on one or more of the features needed by the digital autopilot.
Other existing servos utilize stepping motors. However, the torque capacity of these stepping motors is insufficient for some applications and no disconnect is provided.
Thus, there is a need for improved digital autopilot servos that meet the size, weight, safety, and interface needs of modern general aviation and “home-built” aircraft systems. These needs and other needs are satisfied by the autopilot servos of the present invention.